Method and device for the three-dimensional determination and digitization of a plaster-or positive-model

ABSTRACT

A method for the three-dimensional determination and digitization of a plaster- or positive-model, in particular for the production of replacement teeth, with improved handling and costs. According to the method, a plaster- or positive-model is clamped in a mounting with a defined orientation and irradiated by means of a radiation source; the radiation reflected by the plaster- or positive-model is recorded and evaluated by a scanning unit. Separation information is generated, and defined movement of the plaster- or positive-model relative to the radiation source takes place along an axis (y), approximately perpendicular to the direction of radiation and linking the displacement with the separation information. The invention also relates to a device for carrying out the method.

The invention relates to a method and a device for the three-dimensionalmapping and digitization of a plaster or positive model for producingdental prostheses.

In particular, the invention relates to the field of producing basicstructures for dental prostheses, in particular for dental crowns and/orbridges for fastening to prepared natural and/or artificial tooth stumpsor the like.

A number of devices and methods for producing artificial dental bridgesand crowns are known. Generally, after the dental preparation in whichthe teeth used for anchoring are prepared by grinding for receiving acrown or bridge or for which, e.g. a pin is implanted, an impression ofthe tooth stump, the surrounding area and jaw is made. This is usuallydone with silicone sealing compounds, but other materials are alsoknown.

A so-called master model can be made from the impression (shows thesituation in the patient's mouth negatively) by means of a plaster cast.This model shows the situation in the patient's mouth positively. Inthis model, the dental technician with his handicraft skills fashions amodel of the basic structure of the dental prosthesis from wax or fromplastic which melts at a low temperature or hardens in a polymerizingmanner (positive model). In this case, the dental technician can alsotake the counter occlusion of the other jaw into account by means of theplaster model in hand.

Traditionally, the model produced by the dental technician is embeddedand melted in heat-resistant substances. The basic structure can be madeof in conventional metal dental alloys by precision casting in the moldthus produced.

For cosmetic reasons, a facing in ceramic or plastic is usually alsomade, at least in the area of the front teeth is known from WO 99/47065to completely digitalize the outer and inner surface after a wax model(positive model) has been formed. A model which inadequately reflectsthe situation in the patient's mouth is then mathematically completedwith respect to the three-dimensional outer and inner surface. Theresult of the digitization and a calculated supplementation shouldrepresent a digital description of the complete surface of the basicstructure of the prosthesis to be produced. The positive model canthereby be turned in steps of up to 180° to digitalize the occlusivelyand cavitally accessible surfaces. The digitization described in theembodiment in WO 99/47065 of a wax model (positive model) of a toothbridge construction should take place by sinuous line scanning of thewax body from two sides by the positive model being clamped between twowaves.

The digitization is thereby accomplished mechanically or optically. Forthis purpose, reference is made to methods for digitization in the mouthof a patient on a prepared tooth stump or to models which, for example,are known from U.S. Pat. No. 4,182,312 with respect to a mechanicaldigitization and from EP 0 054 785 A1 with respect to an opticaldigitization.

The fundamental disadvantage of the mechanical digitization known fromU.S. Pat. No. 4,182,312 is in the fixing of the mechanical scanningdevice to the patient, since the scanning is to take place directly inthe oral cavity of the patient. The secure handling of the device in thenarrow oral cavity is equally problematic. A processing machine forproducing dental prostheses should be controlled directly with thescanning of teeth and surrounding tissue as in a duplicating mill.

To this end, a probe having a transmission rod securely fixed to it mustbe moved by the dentist over the surfaces in the patient's mouth thatare of interest. A complete detection of the surface requires very manyscanning movements, which is very stressful for the patient due to thetime needed. Furthermore, the probe tips must be changed, depending onthe shape of the processing tool.

With the method described in EP 0 054 785 A1, an image recording head isto be inserted into a patient's mouth. This image recording head is todetect a three-dimensional image of a tooth cavity or the like. For thispurpose, the image data is to be displayed on a computer screen, so thata dentist can check to see whether the positioning of the imagerecording head enables a sufficiently accurate image. If necessary, themore favorable positioning of the image recording head can be changedaccordingly.

When a proper position has been obtained, a three-dimensional image ofthe tooth cavity or the like should—without further explanation—beformed spatially true to size. The appropriate data is then to becompleted by interpolation and manual processing of the data set alongthe lines of a CAD construction, until a corresponding dental prosthesisbody has been completely formed. The corresponding data should then beused to work on a suitable blank in order to produce a suitable dentalprosthesis directly from the image while avoiding the aforementionedskilled production steps.

The awkward manipulation with the camera in the patient's mouth was alsofound to be disadvantageous in practice with this method, and inparticular, it requires great discipline on the part of the patient.

Furthermore, as described in the aforementioned document, it isnecessary to coat the tooth which is to be mapped with a powder toobtain defined reflection conditions, since the natural dental materialhas translucent properties. Due to the translucent properties, lightcould otherwise penetrate partially uncontrolled into the tooth stump tobe measured and perhaps be reflected in deeper layers which would resultin an inaccurate result. However, the coating with a reflection powdersimultaneously increases the inaccuracy by the application of the powderwhich will inherently and, based on the restricted conditions in thepatient's mouth, always be irregular in practice. The limited resolvingpower of the image recorder and the difficult lighting conditions in themouth to be mapped are also disadvantageous.

Furthermore, other methods for the optical digitization of workpieces inthe field of dental technology are also known in which a clampedworkpiece is shown in typically 8 to 16 different angular positions andthe data thus obtained is mathematically compiled to form a volumemodel. In addition to high demands for accuracy of the devices used,this method causes substantial computing requirements with considerablesources of error due to the many and, thus, long measurements. On thewhole, therefore, these methods are very expensive and time consuming.

A method and an arrangement for the non-contact three-dimensionalmeasurement of denture models is known from DE 43 01 538 A1. For thispurpose, the object to be measured is placed on a rotary table in orderto measure it according to the triangulation principle.

A drill template for implanting artificial teeth by means of CAD/CAMtechnology is produced by laser scanning of a working model according toDE 100 29 256 A1.

A machine tool as well as a method for producing basic structures fordental prostheses is known from WO 01/39691 A1. For this purpose, adental preparatory model of the basic structure is preferably scanned ina tactile manner to produce, from the digitization data therebyobtained, a blank for producing the basic structure. For the scanning,the preparatory model can be set in two positions turned by 180°.

The object of the invention is to provide an improved method, especiallywith respect to handling and cost efficiency, for mapping a plaster orpositive model and a device for carrying out the method.

According to the invention, the object is essentially solved by:

-   -   clamping the plaster or positive model in a mounting which is        rotatable about an axis of rotation in a defined orientation;    -   irradiation of the plaster or positive model by means of a        radiation source and receiving the radiation reflected by the        plaster or positive model;    -   evaluating the reflected radiation by a scanning unit and        generating a distance information;    -   defined movement of the plaster or positive model relative to        the radiation source along a plane and/or first axis (y) which        extends perpendicular or almost perpendicular to the direction        of radiation;    -   linking a signal for detection of the rotation with the path and        distance information for forming a three-dimensional volume        model of the plaster or positive model, whereby the distance        between the mounting and scanning unit in direction of the        optical axis (z axis) of the scanning unit remains unchanged or        essentially unchanged during the digitization of the plaster or        positive model along a scanning path s, where s≧1 mm.

In particular, the scanning path s corresponds to the entire or almostthe entire scanning distance along a side of the model to be scanned. Aturning is not required for the measurement and digitization of aplaster model. With a positive model, it is necessary to turn it byapproximately 180° about the axis of rotation of the mounting, whichextends perpendicular to the direction of radiation.

With the method according to the invention, very accurate results can beobtained with relatively simple constructions, in addition to which themethod is not very prone to error sources. Furthermore, thecomputational effort for forming a data model of the body to be mappedis much less compared to the known methods since a plurality ofdifferent views no longer have to be mathematically interlinked, giventhat the positive model is measured in only two positions displaced by180° and the plaster model only in one position. Furthermore, thedistance between the mounting or the plane mounted by it or a planepassing through the axis of rotation and the scanning unit, inparticular when scanning a side, remains constant or almost constant, atleast however in a direction of scanning along the plaster or positvemodel or along a scanning path.

A software for realizing the data processing can be createdsubstantially more easily with this measure, as a result of which thespeed of operation increases, the hardware requirements are reduced infavour of a more advantageous price and, due to the simpler structure ofthe software, the danger of programming and calculating errors isconsiderably reduced.

In an especially advantageous embodiment, the radiation is performed bya line scanner. By designing the method of the invention in this manner,only a mechanical movement in one axis has to be carried out for thethree-dimensional digitization of a body or its surface, as a result ofwhich the equipment requirement can be even further reduced and loweredin cost and, at the same time, possible errors are reduced by mechanicaltolerances.

In particular due to the currently still moderate resolving power ofline scanners, however, it can also be advantageous for obtaining a highaccuracy if the radiation takes place by a laser with an almostpoint-like beam.

In this case, it is especially advantageous if the method also comprisesthe step of a defined movement of the body relative to the radiationsource along a second axis almost perpendicular to the direction ofradiation and linking the second travel path with the scanninginformation and the first path.

To determine absolute values of height information of the body, it isadvantageous if the distance information is standardized to a referencepoint of the body, in particular, if the reference point is almost thatpoint of the body which delivers the lowest distance value, which isadvantageously ascertained by a preliminary run-through of the method.Absolute information about the height of the body can be read directlyfrom the data thus obtained which can e.g. be used to select the blank.

Furthermore, according to the invention, the object is solved by adevice comprising a mounting for accommodating a plaster or postivemodel to be mapped and digitalized, a scanning unit for opticallyscanning the plaster or positive model, the mounting with the plaster orpositive model being diplaceable in at least one direction relative to,and at a right angle or almost at a right angle to, the optical axis ofthe scanning unit, and a device for detecting the path of the mountingwith the plaster or positive model which is rotatable at a right angleabout the optical axis by at least 180° or almost 180° in the at leastone direction, whereby the scanning unit comprises a CCD image recorder,a birefractive crystal and an objective in the ray path. The possibilityof turning the mounting is not required when measuring and digitalizinga plaster model.

In an especially advantageous embodiment of the device, the scanningunit also comprises a laser diode as well as a device for the imaging ofthe light of the laser diode into the path of rays of the scanning unit.

Further details, advantages and features of the invention can be foundnot only in the claims, in the features to be found therein—separatelyand/or in combination—but also in the following description of thepreferred embodiments found in the drawings, in which:

FIG. 1 shows a schematic view of a device for carrying out the methodaccording to the invention;

FIG. 2 shows the device from FIG. 1, in which each of the mountings havebeen omitted, so that the optical scanning unit and the cutting tool canbe seen; and

FIG. 3 shows a simplified side view of the device of FIGS. 1 and 2.

The orientations of a coordinate system noted in the following relate tothe illustration in the attached drawings and serve only to describe theinvention.

If the invention is described essentially with reference to a positivemodel, this does not, however, restrict the invention. The same appliesanalogously to a plaster model.

In the first embodiment, a positive model in the form of a wax model 1of a dental bridge is clamped in a mounting 2 of a device according tothe invention, as shown in the figures. The mounting 2 is mounted on ashaft 3 which enables a rotation of the mounting 2 by 180°. Furthermore,the shaft 3 is mounted on a table 4 which can travel precisely in threeaxes x, y, z. The axis of rotation of the shaft 3 extends, for example,in the y direction. The drive of the table 4 is mounted in an equipmenthousing 5. The opening in the equipment housing 5 required for thetravel movement of the table 4 can be covered in any known manner, e.g.by a bellows or by a sleeve 6.

Furthermore, an optical scanning unit 7 is accommodated in the equipmenthousing 5 for measuring distance. The scanning unit 7 comprises a laserbeam source (not shown in greater detail), e.g. a laser diode, as wellas advantageously a device for reflecting the light of the laser diodeinto the ray path of the scanning unit 7 and further optical elements aswell as a CCD camera adjusted in its sensitivity to the laser. Abirefractive crystal which splits the laser light reflected by the waxmodel 1 (positive model) into a regular portion and an irregular portionis arranged in front of the CCD camera, as a result of which hologramswith border areas are generated on the CCD image recorder which can beaccurately measured and with reference to which the exact distance tothe measured point can be determined.

The scanning unit 7 is fastened in the equipment housing 5 in such a waythat an emitted laser beam runs along the z axis. After a singlecalibration during assembly, the scanning unit 7 delivers absoluteinformation about the distance to an object reflecting the laser beam,e.g. a wax model 1 (positive model) clamped in the mounting 2, accordingto the so-called conoscopic holography. Details of this measuring methodare described, for example, in WO 99/64916, U.S. Pat. No. 5,953,137, WO99/42908, U.S. Pat. No. 5,892,602, U.S. Pat. No. 5,291,314, EP 0 394137, EP 0 394 138 and U.S. Pat. No. 4,976,504.

The high intensity of the laser light enables the use of an imageforming objective with a relatively small opening, so that a field depthis produced which is larger than, for example, the typical height of adental bridge or the wax model 1 thereof or the dental stump of aplaster model, e.g. 15 mm.

Since the previously described scanning unit 7 gives measured valuesabout the absolute distance of the point lit by the laser beam based onthe reflection as measured value, when mounting a device according tothe invention, not only is the scanning unit 7 adjusted such that thelaser beam is parallel to the z axis of the table 4, but the scanningunit 7 is also calibrated via a reference plate which is clamped in themounting 2. Thus, the area of the tolerable blur (field depth) canthereby be simultaneously determined by moving the table 4 accordinglyin the z direction.

During a later mapping of the positive model 1 such as the wax model orplaster model, the mounting 2 is moved over the table 4 along the z axisof the table 4, accordingly in the focus range of the scanning unit 7.The plaster or positive model 1 is now digitalized by moving themounting 2 and the table 4 in a defined manner along the x and y axis,e.g. by line or in columns, and this information is linked with thedistance information determined by the scanning unit 7. The position ofthe table 4 and with it of the model 1 to be mapped in the z direction,is subtracted from the distance value which the scanning unit 7 gives toform the measured data set. During scanning of the model 1, the table 4is not moved along the z axis but only in x and y direction.

By linking the x and y position values with the distance information ofthe scanning unit 7, a data pattern is produced which reproduces thethree-dimensional design of the side of the plaster or positive model 1facing the scanning unit 7.

For the complete three-dimensional appraisal of the entire model 1, thepositive model 1 together with the mounting 2 is turned about the y axisthrough 180° after one side has been scanned and the rear side of thepositive model 1 is mapped in the same manner.

However, a prescan (preliminary run-through of the method) can also beundertaken prior to starting the measurement of the first side of thepositive model 1 to determine an extreme value of the positive model 1in the z direction, e.g. the model point with the least distance to thescanning unit 7 and the associated z value of the coordinate asreference value and thus the distance information standardized to themodel point as reference point. This reference value can be adopted forforming a reference plane perpendicular to the z axis. In this way, themaximum extents of the mapped model can be derived directly from thedata set generated.

If redundant measured data is generated by the mapping of two sides,these can be removed later by appropriate reprocessing by software whenforming the volume model to avoid malfunctions during later control of aprocessing machine or a processing tool such as a milling tool 8.

A milling tool 8 of this type is advantageously integrated in a housing5, for example, relative to the table 4, opposite the optical scanningunit 7. Advantageously, the milling tool 8 has a stationary spindle. Aceramic blank 9, for example, consisting of a presintered yttrium oxidestabilized zirconium oxide, is clamped in a further mounting 10 which isconnected with the rear end of the shaft 3. The forward movements in thex, y and z directions required for processing the side of the blank 9facing the milling tool 8 are carried out by corresponding movement ofthe table 4 with the shaft 3 and the mounting 10. When the processing ofthe side of the blank 9 facing the milling tool 8 is finished, the blank9 can be moved away from the milling tool 8 in z direction by a forwardmovement and the mounting 10 turned by 180°, as during scanning of thepositive model 1, to process the other side of the blank 9.

Instead of a ceramic blank 8, a blank consisting of any other suitablematerial, e.g. a metal, plastics or composite materials, can also beused.

In a further embodiment of the invention (not shown in the figures), theuse of a so-called line scanner is provided instead of the laser beamwith almost point-like cross section, whereby the line width shouldcorrespond to at least the width of the model to be scanned, e.g. in theorder of 100 mm. With a line scanner of this type, which can, moreover,work similarly to the scanning unit 7 already described above, it wouldthen be possible to completely digitalize three-dimensionally a positivemodel 1 or also a plaster stump or a plaster model of the jaw by movingthe table 4 with the mounting 2 and the plaster or positive model 1along an axis. If a positive model is scanned, both sides are measuredby turning the mounting through 180°. With a plaster model, only theside with the dental stumps is scanned.

For example, a wax model of a bridge construction, which hasthree-dimensionally formed functional or connecting surfaces both on theupper side and on the lower side, scanned from both sides by a linescanner of this type after a rotation of the mounting through 180°, aspreviously noted.

The scanning of a three-dimensional plaster or positive model 1according to the invention by displacement along only one or at most twoaxes, with an additional turning of the model 1 through 180°, alsorepresents, with respect of the computational effort required to form athree-dimensional data model of the measured object, significantprogress compared to the known optical scanning devices, in which theobject to be scanned is usually tilted several times and the datapattern of the various “views” thus obtained must be linked with oneanother by appropriate computational operations to produce a volumemodel of the measured object.

However, care must be taken that, for a sufficiently reliable reflectionand thus a reliable distance adjustment by the scanning unit 7 withtypical materials for the modelling in the dental field, the surfaces tobe mapped form an angle of at least about 0.1°, preferably of at least1°, to the z axis with the optical axis of the laser beam. Nevertheless,the angle should not exceed 20°. However, this does not represent alimitation in practice since, at the latest for mounting the dentalprosthesis onto the prepared tooth stump or the implant, at least suchan inclination is required for the proper cementing of the prosthesis aswould be required for a shape inclination of a conventional castprosthesis. Undercuts may not occur in any event in prostheses of thesetypes, since cavities between the prosthesis and tooth stump could formin this case, which would inevitably lead to further damage of the toothstump, for example, by caries bacteria remaining in the cavity thusproduced.

To ensure a sufficiently accurate clamping of the plaster or positivemodel 1 to be mapped in the mounting 2, this can, for example, beaccomplished with aid of a parallelometer, in which an apparentundercut, caused by an inclined position of the model or a tangentialrun of the laser beam of the scanning unit 7, can be prevented with verygood reliability and reproducibility with aid of the adjustment of theso-called light-gap method.

It is understood that it can also be provided that the scanning unit 7moves in the z direction instead of the table 4 being moved in the zdirection or even that a movement in the z direction can be entirelyomitted for the scanning if the scanning unit 7 is equipped withinterchangeable objectives of various focus lengths for adapting theworking distance or with a zoom optic having an adjustable focal length.

1. Method for the three-dimensional mapping and digitization of aplaster or positive model (1), for producing a dental prosthesis, withthe following steps: clamping the plaster or positive model (1) in amounting frame (2) which is rotatable about an axis of rotation in adefined orientation; irradiating of the plaster or positive model (1) bymeans of a radiation source and receiving the radiation reflected by theplaster or positive model (1); evaluating the reflected radiation by ascanning unit (7) and generating a distance information; positioning themounting frame in a plane running parallel to the x- and y- axes of aCartesian coordinate system; definedly moving the plaster or positivemodel (1) relative to the radiation source along a first axis (y) whichextends perpendicular or almost perpendicular to the direction ofradiation; definedly moving the mounting frame (2) with the plaster orpositive model relative to the radiation source along a second axis(x-axis) extending both perpendicularly or almost perpendicularly to thefirst axis (y-axis) and perpendicularly or almost perpendicularly to thedirection of radiation; linking the second travel path with the distanceinformation and the second travel path; linking a signal for detectionof the rotation with the path and distance information for forming athree-dimensional volume model of the plaster or positive model (1),whereby the distance between the mounting (2) and scanning unit (7) indirection of the optical axis (z axis) of the scanning unit (7) remainsunchanged or essentially unchanged during the digitization of theplaster or positive model (1) along a scanning path, where the length ofthe path is greater than or equal 1 mm.
 2. Method according to claim 1,characterized in that the distance between the mounting frame (2) andthe scanning unit (7) along the entire or almost entire scanning path sof a side of the plaster or positive model is constant.
 3. Methodaccording to claim 1, characterized in that, to determine and digitalizethe plaster or positive model, it is turned through 180° orapproximately 180° about the axis of rotation of the mounting whichextends perpendicular to the direction of radiation.
 4. Method accordingto claim 1, characterized in that the radiation takes place by a linescanner.
 5. Method according to claim 1, characterized in that theradiation takes place by a laser beam with almost point-like beams. 6.Method according to claim 1, further characterized by the followingstep: standardizing the distance information to a reference point of theplaster or positive model (1).
 7. Method according to claim 6,characterized in that the reference point is almost that point of theplaster or positive model (1) which delivers the lowest distance value.8. Method according to claim 6, characterized in that the referencepoint is determined by a preliminary run-through of the method.